This invention relates to methods of manufacturing elongate members and has particular reference to methods of manufacturing elongate members of cellulose, further particularly but not exclusively cellulosic fibres.
Methods of producing cellulosic elongate members such as fibres and films are well known.
Cellulosic fibres are formed from polymer molecules consisting of large numbers of anhydro glucose units joined together. Some cellulose fibres are natural, such as cotton; other cellulosic fibres, such as rayon, are produced by regeneration from fibres of vegetable origin such as wood.
Viscose rayon fibre is a regenerated cellulosic fibre produced by the treatment of cellulose by caustic soda and subsequent xanthation to form sodium cellulose xanthate as an intermediate chemical compound. This compound will dissolve in caustic soda and produce a viscose dope. The viscose dope consists of the chemical compound of cellulose, i.e. the sodium cellulose xanthate, in solution in the caustic soda. After filtering, the viscose dope is extruded or spun into an acid bath to produce the fibre. In the acid bath the sodium cellulose xanthate decomposes to regenerate the cellulose.
More recently, processes have been discovered in which the cellulose can be dissolved directly in a solvent without the formation of an intermediate chemical compound. A solution of cellulose in certain solvents, for example tertiary amine-N-oxides (herein referred to as amine oxides), will produce a dope which can be spun directly into a spin bath to form by coagulation an elongate member such as a fibre by dissolving the solvent in the water of the spin bath and recrystallising or precipitating the cellulose. This type of process is referred to herein as "solvent spinning".
After the spinning step in solvent spinning, the fibre is passed through a series of water baths to remove the residual amine oxide still in the cellulose and through bleach and wash baths to produce a cellulosic fibre from which the amine oxide has been removed virtually completely. After the wash baths, the fibre is dried in a conventional drying oven to produce a tow for subsequent processing.
The direct production of cellulosic elongate members, such as fibres, using solvent spinning rather than viscose production has some commercial advantages over the viscose route in that there is much less chemical usage and the equipment required is simpler. However, it has been found that cellulose fibre produced by such a direct solvent-spinning route does have properties different from regenerated cellulose fibre produced by the viscose production process. In particular, it has been found that solvent-spun cellulosic fibre suffers from fibrillation when wet-abraded.
Fibrillation comprises the partial breaking up of the fibre in a longitudinal direction with the formation of small hairs on the fibre. These hairs tend to twist and give the fibre, when looked at under the microscope, a hairy appearance. These hairs, or fibrils, on the fibre give rise to two significant problems; the first problem is the appearance of the fabric and the second problem is the tendency of the fabric to form pills on its surface.
The fibres can be dyed loose, or fabrics woven from undyed material can be dyed in one of two ways. The fabric can either be open-width dyed, which means that the material is dyed on a continuous basis, or the fabric can be rope dyed, which means that the fabric is dyed in a batchwise exhaust-dyeing process.
Each process has its own advantages and disadvantages. Many fabrics are dyed by rope dyeing, which means that small quantities of the fabric can be dyed, and there is less wastage and down-time in the process. Essentially, the fabric is contracted into a rope and then passed into a vat for dyeing purposes. It has been found that, if a fabric is woven or knitted from a solvent-spun cellulosic fibre produced from the direct dissolution of the cellulose in the solvent, then fibrillation occurs during the rope-dyeing process. After dyeing, the fabric has a white-looking surface, a fibrillated or frosted finish which is unacceptable in many cases, particularly if the fabric is dyed to a dark colour, such as dark navy or black, when the fibrils show up as a light white frosting on a dark background. Further washing of the fabric after use can then make the fibrillation effect worse until the garment made from the fabric is visually unacceptable, although physically quite useable.
Although it is possible to open-width dye woven fabrics, it is often not practical to open-width dye knitted fabrics because of the need for relaxation during the dyeing process. Dyeing in the open-width process tends to give a tenser, harsher fabric than rope dyeing.
As is mentioned above, material which has fibrillated during the dyeing process tends to fibrillate further during washing processes, and after repeated washing the fibrils ball up and pill. Because cellulosic fibres formed from solvent systems are inherently strong, the pills are held onto the fibres and do not fall off the fabric. Again this can reduce the attractiveness of fabrics and garments made from fibrillated fibres.
There is therefore a need to produce a way of reducing the tendency to fibrillation in solvent-spun cellulose fabrics produced from a cellulose dope made by the direct dissolution of cellulose in a solvent.